This invention relates to methods of joining metal surfaces as by brazing, welding or the like. More particularly, the invention relates to improvements in fluxing, i.e. the provision of flux, in metal-joining operations, and to flux compositions having utility therein, as well as to methods of making such fluxes.
Stated broadly, the present invention is applicable to metal-joining techniques wherein metal surfaces are joined with metal heated to molten state at temperatures above about 560.degree. C, and wherein a flux is employed for such purposes as cleaning the surfaces to be joined and/or promoting flow of molten filler or joining metal. The welding and brazing of a variety of metals are thus appropriate for the practice of the invention in this broad sense.
In one important specific aspect, the invention is particularly directed to improvements in the brazing of aluminum and alloys thereof, as affording especially significant advantages in overcoming certain problems heretofore associated with such brazing. Detailed reference will be made herein to aluminum brazing operations, for purposes of illustration, and as exemplary of the types of operations with which the invention may be used.
The brazing of aluminum involves joining surfaces of aluminum or aluminum alloy components with a so-called aluminum brazing alloy, i.e. an alloy of aluminum typically having a melting point substantially lower than that of the components. In such operations, the component surfaces are joined by juxtaposing them with the brazing alloy adjacent to or between them, and heating to a temperature that will effect melting of the brazing alloy without melting the components. For assured attainment of this heating condition, it is commonly preferred in present-day industrial practice that the melting point of the brazing alloy be at least about 30.degree. to about 40.degree. C lower than that of the components. A currently preferred brazing alloy is an aluminum-silicon eutectic melting at about 577.degree. C. It will be understood that reference herein to "surfaces to be joined" in aluminum brazing operations means the surfaces of metal (i.e. aluminum or alloys thereof, at the locality of the joint) other than the brazing alloy, which is of course also joined to these other metal surfaces by the brazing operation.
A brazing alloy may be placed adjacent to or interposed between the surfaces to be joined as a discrete sheet or body or in particulate form, or a layer of the brazing alloy may be pre-clad on at least one of the surfaces. For convenience, the term "brazing sheet" will be employed herein to designate articles having a core of aluminum or aluminum alloy with a cladding (on one or more surfaces) of a brazing alloy of lower melting point, including sheet, tube and other forms suitable for brazing; and it will further be understood that the term "surfaces to be joined" as applied to brazing sheet designates the surface of core metal underlying the cladding. Such brazing sheet (typically having a core-cladding melting point differential of about 30.degree.-40.degree. C, as stated) is extensively used e.g. in the production of heat exchangers.
In aluminum brazing operations, it is ordinarily necessary to employ a flux to remove aluminum oxide coatings present on exposed metal surfaces (including brazing alloy surfaces) at the locality of formation of the brazed joint, and also desirably to promote flow of the brazing alloy. The flux is thus disposed for contact with all such exposed surfaces; for instance, when brazing sheet is used, the flux is placed between or in proximity to the clad surface of the brazing sheet and the other aluminum surface to which the sheet is to be joined, i.e. prior to application of brazing heat. Stated in general, the flux must have the properties of acting as a flux to dissolve and/or otherwise remove metal (e.g. aluminum) oxides, becoming reactive (i.e. capable of removing the oxide) at the brazing temperatures, and being essentially inert with respect to aluminum at such temperatures. It is also believed that the flux serves to inhibit formation of further oxide. Since fluxes are usually reactive only when at least partially molten, it is generally required that the flux be partly or wholly molten at the brazing temperature, e.g. (in the specific instance of use of the aforementioned preferred brazing alloy) at temperatures not substantially higher, and indeed preferably lower, than 577.degree. C.
The most common commercial flux heretofore conventionally employed for brazing aluminum has been a mixture of predominantly chloride salts, including alkali metal and alkaline earth metal chlorides with minor inclusions of fluorides as well in some cases. These water soluble materials are corrosive to aluminum in the presence of moisture. Consequently at the end of the brazing operation the brazed assembly must be subjected to a cleaning operation (as by washing with water) to remove the water soluble flux. Even so, there are usually inclusions of the flux in the joint which may result in corrosion after a relatively short interval, particularly where the brazed assembly may be subjected to humid conditions. Moreover, it may be difficult or impossible to achieve fully effective cleaning, especially in the case of assemblies of complex configuration.
Various alternative expedients have heretofore been proposed to avoid the difficulties resulting from presence of a water-soluble flux residue at the locality of the joint after brazing. For instance, it is already known to braze aluminum without the use of a flux under vacuum or inert gas conditions, but the capital cost of the equipment employed is very high. Moreover, a major disadvantage of the fluxless brazing methods is that much closer tolerances must be observed for assembly than for flux brazing. Any failure to maintain very close tolerances results in the brazed assemblies being rejected due to incompletely filled joints.
It has also heretofore been proposed, in British Pat. No. 1,055,914, to produce a flux for joining aluminum by mixing 53-55% AlF.sub.3 with 47 to 45% KF (percentages by weight), within which range a known eutectic point occurs, having a melting point of about 560.degree. C. In this prior proposal the materials are either mixed dry, with subsequent addition of water, or the KF is added in aqueous solution. In both alternatives the resultant paste is dried at a temperature below 200.degree. C, and thereafter applied (i.e. without other heating) to the surfaces to be fluxed.
The material produced by that method was reported as leaving a brittle, nonhygroscopic residue at the end of the joining operation. However, prior to the joining operation, the soluble potassium fluoride and insoluble aluminum fluoride in the flux mixture are at least very largely free or unreacted; the flux mixture is thus hygroscopic, and is unsuitable for use in aqueous slurry. Slurrying of this material in water would result in solution of KF and consequent possibility of disproportionation of the flux on drying and melting point variability. For example, if this fluoride mixture were applied in aqueous slurry to a metal surface, run-off of water would cause preferential removal of the dissolved potassium fluoride and thereby upset the desired relative proportions of the two fluorides. Indeed, the aforementioned fluoride mixture usually cannot be satisfactorily stored for any length of time without special protection from the atmosphere, even if initially more or less dry, owing to the effect of atmospheric moisture on this highly hygroscopic material.
Even if (as proposed in the patent) the fluoride mixture is used in an inert and nonhygroscopic vehicle, it may nevertheless be vulnerable to deterioration in the presence of atmospheric moisture, again because of its hygroscopic nature, since many such vehicles (e.g. resins) are more or less permeable to moisture. Moreover, in some brazing operations use of a resinous vehicle may be inconvenient or undesirable. For instance, in production of certain closed heat exchanger assemblies (e.g. radiators for motor vehicles and evaporators for air conditioners) by brazing, particular operating conditions may create a carbonaceous residue of a resin vehicle during brazing; i.e. the available oxygen in such closed assemblies may in particular cases be insufficient, in relation to the amount of resin present, to enable adequate burning off of the resin incident to the brazing operation. In these and other circumstances, then, it is sometimes preferable that the flux be entrained in a vehicle which is substantially completely evaporable. Of such vehicles water is generally the most suitable, both from aspects of cost and operating convenience, since it leaves no residue and requires no oxygen for burn-off.
In other words, then, it would be desirable for many aluminum brazing operations to employ a flux which not only meets the general requirements for aluminum-brazing fluxes, and leaves no substantially water-soluble or corrosive residue after brazing, but which also is nonhygroscopic and substantially water-insoluble prior to brazing. Among the advantages of the last-mentioned property would be capability of storage of the flux, ease and convenience in handling and applying the flux, avoidance of disproportionation of the flux constituents, and ability of the flux to be used in an aqueous slurry.
While the foregoing discussion has been directed specifically to aluminum brazing operations, similar considerations are more or less relevant to other metal-surface-joining operations as well.